Traditionally, in a refrigeration cycle apparatus in which a compressor, an oil separation unit, a condenser, an expansion valve, and an evaporator are connected in the named order, an oil separator is provided at the discharge side of the compressor for discharging refrigerating machine oil along with refrigerant from the compressor. Also, the refrigerating machine oil having been separated from the refrigerant in the oil separator is returned again to the suction side of the compressor. Here, various flow paths and control methods for returning the oil from the oil separator to the compressor are proposed (see, for example, the patent literatures 1 to 3).
Patent Literature 1 discloses a refrigeration cycle apparatus in which a connection pipe including a capillary tube and a flow path, which has an oil tank, a valve, and a capillary tube, are connected in parallel with each other between an oil separator and a suction side of a compressor. Also, opening and closing of the valve is controlled based on a discharge temperature of refrigerant discharged from the compressor and a temperature of a refrigerating machine oil flowing in the connection pipe (or the temperature of the refrigerant taken into the compressor). Patent Literature 2 discloses an air conditioner in which an oil tank is connected via a capillary to an oil separator, and a first circuit having a solenoid valve and a second circuit are connected in parallel with each other between the oil tank and a suction side of a compressor. Also, in activation after non-operation, the solenoid valve is opened and a refrigerating machine oil stored in the oil tank is supplied to the compressor. Patent Literature 3 discloses an air conditioning apparatus in which a first flow path including an expansion device and a second flow path including an expansion device and a solenoid valve are connected in parallel with each other between an oil separator and a suction side of a compressor. Also, opening and closing of the solenoid valve is controlled based on the degree of superheat of the suction side of the compressor or an operation frequency.